This invention relates to an arrangement for controlling the engagement of a chain with a chain sheave (cable lifter, wildcat) by twisting the chain if a link in horizontal position engages the teeth (whelps) of the sheave instead of entering into its pockets. The expressions "horizontal" and "vertical" as applied to chain links are customarily used within the art to indicate the position of the link relative to a chain sheave.
A particularly important use of the invention is for solving the problems arising where a line to be passed around a chain sheave besides a common chain comprises an enlarged link, such as a special coupling or splicing link, requiring to be engaged with the chain sheave in a predetermind position with respect to the pockets and teeth of the chain sheave, and where the fulfillment of this requirement depends on correct orientation of the adjacent chain link, either vertial or horizontal depending on the geometrical conditions.
When the chain has entered the chain sheave with the splicing link in incorrect position, the problem has been solved by turning the chain while this is in engagement with the chain sheave, but this is very difficult and complicated with the great anchor chain dimensions used on ships and oil drilling platforms.
For the splicing of chains there have therefore been constructed relatively complicated so-called Kenter shackles, which can pass the chain sheave in both vertical and horizontal position, the distance between opposed teeth of the sheave being made as great as possible. An ordinary shackle, which is likewise frequently used for the splicing of chains, can only pass such a sheave in horizontal position.
For the interconnection of a chain and a fibre or steel rope, it has not been found possible to construct a coupling having dimensions such as to permit the coupling link to pass between opposed teeth of a chain sheave. For semi-submersible oil drilling platforms, the combination of a chain and a rope as a mooring line has many advantages, but one of the difficulties encountered is that of ensuring correct engagement of the coupling link with the chain sheave during its passage around the latter.
When the rope is hauled in by means of a separate winch, but on its way is guided around the chain sheave in the groove of the latter, the operator will see to it that the coupling link will be landed in a pocket of the sheave, but the orientation of the chain about its longitudinal axis is not under control. After some time the chain will synchronize itself with the chain sheave, but in this process of synchronization the chain will be displaced relative to the circumference of the chain sheave, and therefore when, during pay-out, the coupling link returns to the sheave, it may land on a pair of teeth or in a pocket of the sheave depending on the orientation at which the chain entered into engagement with the sheave during haul-in.